ci: Use remote pull/merge ref instead of local git merge

The merge strategy on the remote may be different than the local one.
This may cause local merges to be different or fail completely. Fix this
by using the result of the remote merge.

(copied from bitcoin/bitcoin@fad7281)

Author: real-or-random

.cirrus.yml

@@ -53,11 +53,11 @@ cat_logs_snippet: &CAT_LOGS
 
 merge_base_script_snippet: &MERGE_BASE
   merge_base_script:
-    - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
-    - git fetch $CIRRUS_REPO_CLONE_URL $CIRRUS_BASE_BRANCH
     - git config --global user.email "[email protected]"
     - git config --global user.name "ci"
-    - git merge FETCH_HEAD  # Merge base to detect silent merge conflicts
+    - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
+    - git fetch $CIRRUS_REPO_CLONE_URL "pull/${CIRRUS_PR}/merge"
+    - git checkout FETCH_HEAD  # Use merged changes to detect silent merge conflicts
 
 linux_container_snippet: &LINUX_CONTAINER
   container: